Science fair project:

"Why do bees die when they've just stung?"

So here's what I learned:  Not all bees sting.  It's mainly the worker bees, who are out, and who sting to protect the nest.  They have barbed stingers.  The stingers get caught in the skin of thick-skinned animals (like humans) but not in thin insect skin.  The bee is usually brushed off or shooed away and when it leaves the stinger and lower abdomen tear off.  The venom pumps and other stinging parts stay active after they get torn off, so more venom is delivered after the bee flies away.  It makes evolutionary sense for the bee to die in order to deliver a worse sting for the purpose of protecting the nest.